Production trends within automotive interior vehicle component assembly plants, for vacuum formed components, involve assembly operations that include as the bonding mechanisms: adhesives, glass reinforced urethane, and low pressure molding. These operations typically involve manufacturing of instrument panels (I/P's), side door panels, transmission covers, rear decks, air bag covers and headliners.
Component parts are composed of a rigid backing material with a decorative surface layer. The most common rigid backing substrates include, acrylonitrile-butadiene-styrene (ABS), acrylonitrile-butadiene-styrene blended with polycarbonate (ABS/PC), acrylonitrile-butadiene-styrene blended with styrene-maleic anhydride (ABS/SMA), polycarbonate blended with styrene-maleic anhydride (PC/SMA), pressed wood fiber, and Noryl.TM. (General Electric) (a blend of polyphenylene oxide (PPO) and hi-impact polystyrene (HIPS)). The most common decorative surface layers are vinyl substrates that include rigid, fabric backed, trilaminate, bilaminate and composite types. Trilaminate and bilaminate expanded vinyls (i.e. foams), as well as the composite vinyls have a coating applied to the back side of the vinyl, generally referred to as a backcoating. This backcoating improves adhesion between the vinyl and the component part. Further, if the vinyl substrates contain plasticizers, the backcoating may limit plasticizer migration. This backcoating may be a crosslinked or uncrosslinked polyurethane, polyvinylchloride/polyurethane (PVC/PU), acrylic or occasionally, an ethylene-acrylic acid based coating.
In typical vacuum forming applications, the adhesive is applied to the rigid substrate. It is preferred not to use an adhesive that requires a primer be applied onto the rigid substrate prior to application of the adhesive. A sheet of decorative expanded vinyl is heated above its memory point at which time the vinyl is brought into contact with the dried adhesive and a vacuum is applied. As used in this application, "memory point" is the temperature at which vinyl can be stretched over a contoured surface but still retain the structural integrity of the vinyl.
The temperature to which the vinyl is heated must be sufficient to allow conformability to the rigid surface as well as cause wetting of and bond formation with the dried adhesive. Subsequent to this, and upon removal of the vacuum, the adhesive must be able to pass the industry performance specifications, such as withstanding testing at 110.degree. C. without allowing the vinyl to bridge recessed or deep draw areas. Also, the adhesive or bonding agent must meet the heat requirements of 82-115.degree. C. for automotive interiors for the instrument panel (I/P), the rear deck, shelf area, and the door panel above seat level. Surface temperatures on the I/P can reach 110.degree. C. and the adhesive is required to maintain performance in high humidity/warm environments, and withstand the effects of cold temperature cycling.
In order to meet this series of requirements adhesive formulators have used one or two component polyurethanes in solvents such as methylene chloride. Recent government regulations concerning exposure to chlorinated solvents, as well as isocyanates, have necessitated the development of water-based alternatives. Aromatic and aliphatic polyurethane dispersions with varying degrees of crystallinity, and crystallizing polyamide dispersions that may be modified with materials such as rapid setting ethylene-vinyl acetate emulsions to achieve the desired balance of properties have recently been made available as alternatives to the chlorinated solvent-based adhesives.
The use of crystallizing polyurethane dispersions for vacuum forming applications has been limited to rigid surfaces that possess some polarity such as ABS. The adhesion is inadequate for lower surface energy plastics such as Noryl. In addition the use of material modifiers for these polyurethane dispersions such as ethylene-vinyl-acetate emulsions have also demonstrated inadequate adhesion to lower energy surfaces.
Crosslinkers such as water dispersible isocyanates and aziridines have frequently been used in polyurethane dispersions to obtain the desired heat resistance. However, the adhesives currently available have not demonstrated the performance characteristics, as described above, of solvent-based adhesives.